In recent years, the increasing demand of biodiesel as a renewable energy resource has led to an overproduction of glycerol-based compounds, which are inevitable by-products of the trans-esterification process for producing biodiesel. Unfortunately, the abundant glycerol-based compounds thus produced not only increased storage burden for the biodiesel producer, but also posed an environmental pollution risk. Therefore, new and efficient industrial processes are eagerly searched to efficiently transform the surplus glycerol-based compounds into higher value-added chemicals.
In this aspect, it has been conventionally known to convert glycerol-based compounds to (poly)glyceryl ethers and their derivatives which, due to their amphiphilic nature and other chemical properties, are used in many industrial applications such as solvents, emulsifiers, laundry and cleaning formulations, dispersants, foaming agents, and ink formulations. Said conversion is usually fulfilled through an etherification reaction of a glycerol-based compound or its derivative, in which hydrophilic glyceryl moieties is incorporated onto a long alkyl chain provided by a hydrophobic alcohol. Traditionally, said etherification is realized by Williamson ether synthesis, which uses toxic and expensive glycerol derivatives (epichlorohydrin, 3-chloropropane-1,2-diol or glycidol) as starting materials and needs a strong base to reach a reasonably selectivity.
As an alternative route of etherification, EP 1958929 A (KAO CORPORATION) Dec. 7, 2006 described a process for producing polyglyceryl ether surfactants, which comprises reacting an alcohol with a glycidol in the presence of a simple metal salt of rare earth element as catalyst. While this process appears to produce a moderate-to-high conversion rate of the alcohol reactant, such is not a direct etherification of glycerol with alkyl alcohol, but rather etherification of glycidol compounds. Compared to glycerol, glycidol compounds are more difficult and expensive to manufacture, and also have an adverse tendency of self-polymerization.
Despite the obvious limitations of indirect (poly)glycerol etherification, using direct etherification of (poly)glycerol compounds to prepare ether has been challenging. This is partly caused by the high viscosity and hydrophilic nature of these glycerol-based compounds, which hinders their interaction with a hydrophobic substrate such as an alkyl alcohol in a chemical reaction. Moreover, since glycerol has three hydroxyl groups with similar pKa, with the presence of hydroxyl groups in the ether product, the selectivity control is made even more complicated.
In the previous research work of this field, Wechhuysen, et al reported direct etherification reaction of biomass-based polyols with long-chain olefins under heterogeneous acidic catalysis, in which good results were obtained for diols. However, very low conversion (˜20%) was given in the case of glycerol, see WECKHUYSEN, et al. Chemical Imaging of Catalyst Deactivation during the Conversion of Renewables at the Single Particle Level: Etherification of Biomass-Based Polyols with Alkenes over H-Beta Zeolites. J. Amer. Chem. Soc. 2010, vol. 132, p. 10429-10439. and WECKHUYSEN, et al. Synthesis of long alkyl chain ethers through direct etherification of biomass-based alcohols with 1-octene over heterogeneous acid catalysts. J. Catal. 2009, vol. 268, p. 251-259.
Lemaire, et al studied an alternative route to etherify glycerol and obtain 1-O-alkyl glycerol and diglycerol ethers, by catalytic reductive alkylation of glycerol and diglycerol with linear aldehydes in the presence of 0.5 mol % of Pd/C under 10 bars of hydrogen using a Brønsted acid as co-catalyst. See LEMAIRE, et al. Selective synthesis of 1-O-alkyl glycerol and diglycerol ethers by reductive alkylation of alcohols. Green Chem. 2010, vol. 12, p. 2189-2195. More recently, the same research group reported the realization of a reductive alkylation of (di)glycerol with bio-sourced fatty acid methyl esters, under an even more stringent reaction condition: 50 bar hydrogen pressure in the presence of 1 mol % of Pd/C and an acid co-catalyst. See LEMAIRE, et al. 1-O-Alkyl (di)glycerol ethers synthesis from methyl esters and triglycerides by two pathways: catalytic reductive alkylation and transesterification/reduction. Green Chem. 2013, vol. 15, p. 786-797.
From our previous work (PCT/CN2012/078114), we have found that it is possible to produce glyceryl ether compounds by a direct etherification of glycerol with alkyl alcohol, using a specific Pickering emulsion condition (emulsion stabilized by solid nanoparticles) and optionally with an acidic catalyst. While this newly discovered process advantageously avoids the need of stringent reaction condition (e.g. high pressure or expensive catalyst) and conveniently uses the inexpensive glycerol compounds as starting material, there is still room to improve its reactant conversion rate and product selectivity. Moreover, when a liquid acid catalyst is used in the etherification reaction, extra recycling steps involving liquid-liquid separation would inevitably increase the production cost.
As such, there remains a need to develop a novel process for realizing a direct etherification of glycerol based compounds with alkyl alcohol, which features a higher reactant conversion rate, product selectivity, mild reaction condition and easy recycling of catalysts.